This invention relates to methods of beneficiating the fermentation activity and/or fermentation products of yeast and to a beneficiated yeast product. More particularly, this invention relates to a method of treating active yeast having a relatively low moisture content by heating the yeast prior to utilization of the yeast in a fermentation process, and to the product of the method.
The present worldwide energy shortage has placed a large emphasis on the need to develop potential energy sources other than depletable oil, gas and coal reserves. One potential alternative energy source which has received substantial attention is the production of alcohol, such as ethyl alcohol, from renewable carbohydrate sources, such as corn, grains and other vegetable matter.
In one general approach to the production of ethyl alcohol, a carbohydrate source is subjected to the biological action of suitable yeast organisms to convert at least a portion of the carbohydrates to ethyl alcohol in a process generally referred to as fermentation. After completion of carbohydrate conversion to ethyl alcohol, the produced alcohol is typically recovered from a fermentation mixture by distillation, and then subsequently utilized, such as for a fuel source, as a chemical reagent or solvent, in a consumable alcoholic beverage, or for other purposes. In the past, much of the effects to increase the efficiency of alcohol production from biological or fermentation processes have been directed to maximizing still efficiency or to isolating particular yeast strains which exhibit desired properties in the fermentation process.
There is still a substantial need, however, for new methods or processes which may increase the efficiency of fermentation processes and thereby reduce the costs of alcohol production.
In accordance with the present invention, it has been unexpectedly determined that the fermentation activity of dried yeast, and therefore the efficiency of a fermentation process, can be significantly enhanced by heating a previously dried, active yeast prior to incorporation of the dried yeast into a fermentation mixture. Yeast suitable for use in connection with the invention include active dry yeast which has been produced by known yeast drying processes. Dried yeast is typically produced by innoculating a nutritive medium with active yeast cells, allowing the yeast cells to grow and multiply, recovering a larger volume of yeast cells from the nutritive medium and then drying the recovered yeast in order to obtain a yeast product that can be economically transported and stored. To preserve the activity of yeast during relatively long storage periods, it is a common practice to reduce the moisture content of the recovered yeast prior to packaging for transportation, storage and/or sale. Thus, it is a common practice to compress the recovered yeast, or to subject the yeast to various drying processes, to reduce the moisture content of the yeast to, for example, about 5 to about 20 percent by weight. In typical yeast drying processes, yeast recovered from a liquid medium is subjected to heated air to reduce the moisture content of the yeast to a desired level. It has been generally recognized, however, that the yeast drying temperatures must be carefully controlled in order not to reduce the activity of the dried yeast. In order to avoid the problems of yeast deactivation, yeast drying processes have generally been limited to temperatures of 30.degree. to 40.degree. C. For example, U.S. Pat. Nos. 1,420,558, 1,643,047, 1,859,250 and 2,111,201 relate to such processes. In general, it has been thought that high temperatures must be avoided in order to maintain dry yeast activity.
In order to decrease drying time, it has been suggested in U.S. Pat. No. 3,962,467 of Burrows that yeast may be dried in a fluidized bed in a process which raises the yeast temperature above 50.degree. C. at the end of the drying process. It is recognized in the Burrows patent, however, that the use of yeast temperatures above 50.degree. C. has to be adopted with some degree of caution and if the yeast is maintained at an undesirably high temperature for too long its activity can be seriously damaged. The Burrows patent teaches that the yeast temperature must be above 50.degree. C. For less than 45 minutes, usually less than 30 minutes and most preferably only for the last 15 or 20 minutes of the drying operation. Thus, the Burrows patent confirms the generally held belief that yeast temperatures and drying times must be carefully limited to avoid yeast damage and deactivation or activity reduction.
In view of the care which must be exercised when drying yeast, it is even more surprising and unexpected that dried yeast can be beneficiated according to the process of the invention by heating the dried yeast to temperatures of about 30.degree. to about 60.degree. C. for at least about 2 hours prior to utilization of the yeast in a fermentation process. Yeast treatment according to the invention has been found to enhance the fermentation activity of the yeast and/or to beneficially alter the products of a fermentation process in which the beneficiated yeast is employed.